Input.GetMouseButtonDown

Description

Returns true during the frame the user pressed the given mouse button.

You need to call this function from the Update function, since the state gets reset each frame. It will not return true until the user has released the mouse button and pressed it again. button values are 0 for the primary button (often the left button), 1 for secondary button, and 2 for the middle button.

using UnityEngine;
using System.Collections;

// Detects clicks from the mouse and prints a message
// depending on the click detected.

public class ExampleClass : MonoBehaviour
{
    void Update()
    {
        if (Input.GetMouseButtonDown(0))
            Debug.Log("Pressed primary button.");

        if (Input.GetMouseButtonDown(1))
            Debug.Log("Pressed secondary button.");

        if (Input.GetMouseButtonDown(2))
            Debug.Log("Pressed middle click.");
    }
}